The iconoclastic English painter Banksy may be the most important artist on the run from police. Or the most creative vandal. A graffiti muralist who satirizes government, authority and society, his unauthorized use of urban buildings as his gallery space obliges him to remain anonymous.

Wearing disguises, he has glued his own art-mocking oil paintings onto the walls of leading museums. In 2006 he smuggled a life-size inflatable replica of an orange-jumpsuited Guantanamo Bay detainee into Disneyland, setting off a major security alert.

Quite the outlaw. Yet prints of his art fetch a fortune at Sotheby's (Angelina Jolie paid nearly $500,000 for one) and his coffee-table books are sold at Urban Outfitters.

Banksy's tightrope walk between secrecy and publicity is a central theme of the documentary "Exit Through the Gift Shop." Thierry Guetta, a French expat living in L.A., meets the aerosol guerrilla through Guetta's cousin, another notable street artist who uses the pseudonym Invader. When Banksy visits California, Guetta becomes his lookout, accomplice and video biographer, amassing a huge library of footage documenting the street-art scene. And then things get really interesting.

Banksy sees Guetta's disastrous attempt to fashion a film from his footage and reassesses him as "maybe just someone with mental problems who happened to have a camera." The artist takes over the documentary and encourages Guetta to kick back and create some art of his own. The impulsive Frenchman, who has learned a lot about self-promotion as Banksy's hanger-on but nothing about self-expression, takes the suggestion to heart. He launches a wildly ambitious show, employing hired hands to execute his "vision" (ripped off from the artists he has met) and opens his show with a massive publicity campaign. Calling himself Mr. Brainwash, he becomes, to the chagrin of Banksy and his comrades, an empty overnight success. Credulous gallerygoers empty their wallets for his derivative dreck.

"Exit Through the Gift Shop," credited as "A Banksy Film," poses some bitingly funny questions about the meaning and value of art. Is it in the eye of the beholder? Is it truth plus beauty? Anything you can get away with? Brainwashing? "I used to tell people they should make their own art," says Banksy. "I'm not going to do that anymore." Though his face is obscured by shadow and his voice distorted to hide his identity, it's clear that he appreciates the irony of his art-world pranks backfiring.